doc: usage: Group all shell command docs into cmd/ sub-directory

Currently all shell command docs are put in the doc/usage root.
Let's group them into cmd/ sub-directory.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/exception.rst

@@ -0,0 +1,66 @@
+exception command
+=================
+
+Synopsis
+--------
+
+::
+
+    exception <type>
+
+Description
+-----------
+
+The exception command is used to test the handling of exceptions like undefined
+instructions, segmentation faults or alignment faults.
+
+type
+  type of exception to be generated. The available types are architecture
+  dependent. Use 'help exception' to determine which are available.
+
+  **ARM:**
+
+  breakpoint
+    prefetch abort
+
+  unaligned
+    data abort
+
+  undefined
+    undefined instruction
+
+  **RISC-V:**
+
+  ebreak
+    breakpoint exception
+
+  unaligned
+    load address misaligned
+
+  undefined
+    undefined instruction
+
+  **Sandbox:**
+
+  sigsegv
+    illegal memory access
+
+  undefined
+    undefined instruction
+
+  **x86:**
+
+  undefined
+    undefined instruction
+
+Examples
+--------
+
+::
+
+    => exception undefined
+
+    Illegal instruction
+    pc = 0x56076dd1a0f9, pc_reloc = 0x540f9
+
+    resetting ...